Parasite Control Programs

Why are parasites such a problem?

Internal worms, lice, mites, ticks and flies drain animals of blood, nutrients and energy. Heavy loads cause weight loss, anemia, poor growth, weak immunity and sometimes death. Young animals, pregnant and nursing mothers and animals under stress are most at risk. Goats and sheep are especially vulnerable to certain worms.

Warm, damp weather often increases parasite pressure on pasture. Your extension office knows which parasites are common in your area and when they peak.

Parasites also depend on the animal. A healthy adult with good nutrition often carries worms without harm, while a stressed youngster can be overwhelmed by the same load. Because of that, breeding from animals that stay healthy under the same pasture conditions can improve your flock or herd over the years.

Why is resistance a worry?

Dewormers kill many worms but not all. The ones that survive pass on their resistance. Over time, products that used to work reliably can stop working. Giving dewormer to every animal on a calendar speeds this up. Once resistance is established, you cannot easily reverse it, and for some parasites few products remain effective.

This is why veterinarians increasingly recommend treating only animals that need it, based on testing and clinical signs, and using the right product at the right dose.

How do I know when to treat?

Fecal egg counts, done by a veterinarian or a diagnostic lab, estimate worm burden. For some species, observation tools such as checking the color of the inner eyelid can flag anemia, but you should learn the method from a professional. Body condition, coat quality and manure give clues as well.

Ask your veterinarian for a plan built on testing. Repeat testing after treatment to check whether the product worked.

Keep a close eye on the animals with the heaviest loads. A handful of animals often carry most of the parasites in a group, and finding them lets you treat only those animals.

• Fecal egg counts before and after treatment.
• Regular body condition checks.
• Treat animals that show need, not every animal automatically.

What can I do with pasture?

Pasture management lowers the number of infective larvae animals pick up. Rotating animals to rested paddocks, avoiding overgrazing so that animals are not eating close to the ground, grazing different species in sequence where suitable and not spreading fresh manure on grazed pasture all help. Mixed grazing can break parasite cycles because many parasites are species specific.

NRCS and extension staff can help with grazing plans. Rest time that works depends on climate and season, so ask locally.

Remember that grazing the same ground repeatedly builds up larvae. If you can rest a paddock for a full growing period, or follow animals with a different species, you cut the number of worms waiting on the grass.

How do I use dewormers responsibly?

Use only products labeled and appropriate for your species, and dose by actual weight, not a guess. Underdosing breeds resistance, and overdosing risks harm. Follow label directions for withdrawal times if animals provide meat or milk. Some products can be unsafe for certain species or for pregnant animals.

Keep new arrivals separate and check them before they join the herd. Treating new animals and holding them off pasture for a period prevents importing resistant worms.

What about external parasites?

Lice, mites, ticks and flies cause itching, hair loss, anemia and stress, and some carry diseases. Good nutrition, clean bedding, and regular checks help, and your veterinarian can recommend treatment suited to the problem and species. Be cautious with pesticides, since many are toxic to people, pets and wildlife if misused. Follow the label, and your state department of agriculture can answer rule questions.

Ticks can also spread disease to people. Wear protective clothing, check yourself after working outside and talk to a doctor about any illness after a tick bite. The CDC offers information.

How do I keep a program going year after year?

Keep records of tests, treatments and results for each animal. Over time, you will see which animals carry heavier loads, and you can consider that when choosing which to breed. Review the plan with your veterinarian once a year and adjust to weather and herd changes.

A good program is a bit of work every month rather than a rush of treatment when an animal crashes.

Ask local producers and your extension office about regional resources. Some universities and veterinary schools run workshops on parasite testing and may offer low-cost fecal checks.

Last reviewed: November 12, 2026. Written by the Rural Farm Life Editorial Team. See our editorial policy.